1. Field of the Invention
The present invention relates to an information recording device which magnetically records information consisted of a plurality of bits by a PPM recording method.
2. Related Background Art
The PPM (pulse position modulation) recording method is a per se known method of magnetically recording data upon a magnetic recording medium. FIG. 5 shows signal waveforms used in the PPM recording method.
In the PPM recording method, both data and a clock are transmitted through a single line, and therefore the PPM recording method is also called the self-clock method.
The PPM recording method is designed to determine whether a signal level is "0" or "1" depending on the event occurrence position of the signal waveform. More specifically, the signal of which the event occurs within a half period from the head of the signal period is determined to be "0", and the signal of which the event occurs after the half period is determined to be "1". In FIG. 5, for example, the signal of which the trailing edge of the single waveform occurs at a position approximately one quarter period from the head of the signal period is determined to be 0 (i.e., called a PPM signal of 25% bit location), and the signal of which the trailing edge of the signal waveform occurs at a position approximately three quarter period from the head of the signal period is determined to be 1 (i.e., called a PPM signal of 75% bit location).
The above method enables data to be transmitted correctly even if the trailing edge position in the signal period is somewhat shifted due to a delay over the data line or other cause. When data of "0" is to be transmitted in FIG. 5, it is recognized as data of "0" so long as the signal waveform drops before its half period, even if the data that should drop at one quarter period from the head of the signal period is lagged to actually drop behind the quarter period due to a delay over the data line or other cause. As a result, an error in transmission of data is less likely to occur.
Meanwhile, there are known cameras of the type magnetically recording photographic information, such as the date and exposure data, on a film. Such magnetic recording can be practiced in two ways; i.e. , a method of moving a magnetic head with the position of a film, as a magnetic medium, kept fixed, and a method of moving a film with a magnetic head kept fixed. Because a film is always wound on in cameras each after taking a photograph, the latter magnetic recording method is generally employed by utilizing movement of the film during the winding-on thereof.
When photographic information is magnetically recorded on a film by using the above-described PPM recording system, the photographic information is required to be converted into serial data before start of the magnetic recording. At this time, it is usual to perform the conversion into serial data in accordance with a CPU system clock in a camera. However, as changing a frequency of the CPU system clock is not easy, the serial data cannot be output at an arbitrary clock frequency and hence the magnetic recording can not be made with any desired magnetic recording density.
Further, because the film winding speed is varied depending on the power capacity of a battery loaded in a camera, a film is wound at a lower speed if the remaining power capacity of the battery is small. But an ordinary CPU is designed to output the serial data at a constant speed irrespective of the film winding speed. Therefore, if the remaining power capacity of the battery is small, the photographic information is magnetically recorded on a film more closely than usual. This gives rise to a possibility that the magnetically recorded information may interfere mutually and can not be correctly reproduced in some cases.